1.Laboratory for Climate Studies,National Climate Center,China Meteorological Administration;2.Department of Atmospheric Science,School of Environmental Science,China University of Geosciences
Urbanization has an important influence on the frequency and intensity of heat waves, but the mechanism of urbanization affecting the process of high temperature is not fully understood. The Weather Research and Forecasting (WRF) model is used to simulate a summer high-temperature process in Beijing from 2 to 6 July 2010. This paper reports the main results of urbanization effect on surface air temperature of urban areas in the heat wave process. It is found that the optimized WRF model is able to simulate the temporal characteristics of the 5 consecutive days of high temperature and the variation of IUHI (urban heat island intensity) in Beijing. The impermeability of urban underlying surface determines that the 2m relative humidity of urban area is lower than that of rural area, which weakens the ability of urban area to regulate surface air temperature through latent heat. After sunset, the urban sensible heat flux decreases slowly, and cooling rate of urban area is less than that of rural area. At night, the structure of boundary layer is stable and the height of it is low, and the wind speed is small. In this case, the energy transmission between urban and rural areas is restrained, forming a strong urban heat island at night. After sunrise, sensible heat flux and latent heat flux of urban and rural land surface rise rapidly, and the stability of boundary layer decreases. In the afternoon, the urban underlying surface is in favor of the high and low value centers of sensible heat flux and latent heat flux respectively, with the ability to regulate temperature through latent heat weakened. It’s conducive to the energy vertical transfer that the stability of the boundary layer is weak. The IUHI in the afternoon is smaller than that in evening. The extreme high temperature occurred on 5 July. An abnormal continental warm high pressure controls most parts of China, and Beijing is in front of the high-pressure ridge. The Fohn effect of the northwesterly flowing over the mountains aggravated the urban high temperature except for the larger IUHI. In this process of heat wave, therefore, the obvious urban heat island effect created by the urban underlying surface in Beijing has increased the strength and severity of the extreme high temperature event.